The present invention relates broadly to communications cable and, more particularly, to communications cable containing at least one twisted pair of insulated conductors.
Insulated conductors such as those used in communications cable are often provided as twisted pairs of insulated conductors having two insulated conductors twisted, or xe2x80x9ctwinnedxe2x80x9d, about each other to form a dual conductor group. A typical assembly for these communications cables comprises two or more twisted pairs of insulated conductors xe2x80x9cbunchedxe2x80x9d together (i.e., further twisted and in some instances captured with a binder thread or cable) and contained in a cable jacket. The twisting and bundling of the conductors can facilitate the installation of the cable and connection between insulated conductors. Twisted pair conductors are commonly used in applications such as local area network (LAN) cables and wireless cable network architectures.
One problem associated with communications cable produced with the conventional twisted pair assembly is that crosstalk can occur between twisted pairs of insulated conductors that can negatively affect the signals transmitted by these conductors. Crosstalk may especially present a problem in high frequency applications because crosstalk may increase logarithmically as the frequency of the transmission increases. Some twisted pairs are sufficiently impacted by crosstalk that insulating spacers are positioned between pairs within the same cable. See, e.g., U.S. Pat. No. 5,969,295 to Boucino et al. Another technique for adjusting crosstalk performance involves twinning the conductors of different pairs so that they have different lay lengths and carefully selecting the lay length for bunching.
The insulation employed for conductors is typically a polymeric material. Exemplary insulating materials includes but are not limited to, polyvinylchloride, polyvinylchloride alloys, polyethylene, polypropylene, and flame retardant materials such as fluorinated polymers. Exemplary fluorinated polymers, include but are not limited to, fluorinated ethylene-propylene (FEP), ethylenetrifluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), perfluoroalkoxypolymers (PFA""s) like tetrafluoroethylene and perfluoropropylvinylether (e.g., Teflon PFA 340), and mixtures thereof.
In an effort to reduce the weight and cost of insulation, conductors with foamed polymer insulation, and particularly foamed FEP insulation, have been constructed. The foaming process introduces air into the dielectric medium. Air having a lower dielectric constant increases the velocity of propagation (Vp). Higher Vp typically translates to improved signal transmission speed for high speed data or communications systems. However, the resulting foamed medium tends to become more susceptible to crushing during the twinning and bunching processes. Such crushing can undesirably raise the capacitance and lower the impedance of the finished cable, which can consequently degrade attenuation performance. In order to provide foamed dielectric insulation with sufficient crush resistance to provide adequate cable performance, additional dielectric material has been required, thereby negating some or all of the weight, cost and performance advantages of using a foamed dielectric. Accordingly, it would be desirable to provide a cable having a foamed dielectric with acceptable performance properties while reducing material weight and cost.
The present invention is directed to a communications cable and an associated manufacturing method therefore that can utilize foamed insulators for electrical conductors and still provide acceptable performance. According to certain embodiments of the invention, a communications cable comprises: an elongate cable jacket having an internal cavity; and a plurality of twisted pairs of insulated conductors disposed in the internal cavity of the cable jacket, each of the conductors being insulated with a polymeric layer. Each of the insulated conductors within each of the twisted pairs of conductors defines a twinning helix having a first rotative direction, and each of the twisted pairs defines a bunching helix having a second rotative direction, the second rotative direction being opposite that of the first rotative direction. In this configuration, the communications cable can provide acceptable crosstalk and attenuation performance, even with foamed insulators that have demonstrated unacceptable performance when twinned and bunched in the same rotative direction.
It is preferred that at least one, and more preferably all, of the polymeric layers are formed of a foamed polymeric material (as used herein, a xe2x80x9cfoamedxe2x80x9d polymeric material means both foamed and foam skin materials). It is also preferred that the twinning helices have different lay lengths, and the bunching helix also has a different lay length.